In digital communication links over a channel subject to fading or interference, it is quite common to include multiple bit rates and to change the bit rate based on the current channel conditions. One common example is a telephone-line dial-up modem. The baud rate is determined by the instant telephone line performance. If the line condition worsens, the service is interrupted and a new bit rate is negotiated. It is desired to provide similar capabilities to digital microwave links operating at speeds of 1 gigabit/sec and beyond at frequencies above 10 GHz. In addition, since those links may carry high speed information critical to a business operation, the down time should be kept to minimum and it is desired to reduce the bit rate when a link deteriorates before service outage, and to switch back to higher speed automatically whenever the link condition allows. Furthermore, it is desirable to perform such rate switching with minimum loss of data. Wireless links at these speeds and these frequencies require rate adaptation solutions that respond to the rain fading characteristics of the link and to the required fast switching response time, as solution available for lower frequencies and lower bit rates are inadequate or to costly to implement.
Digital microwave radio links offer an alternative to fiber-optics and other land-based transmission lines whenever the land-based link is not feasible for cost, time or right-of-access reason or simply as an emergency backup when land-based links fail. The Ethernet hierarchy, currently Fast Ethernet (FE) with a throughput of about 100 megabits/sec and Gigabit Ethernet with a throughput of about 1 gigabit/Sec (GigE), is a very popular standard for interfacing such digital links. Higher speed Ethernet of about 2.5 gigabits/sec and 10 gigabits/sec are currently at an early phase of adoption. Digital wireless links may be subject to interference from other communication equipment, however chances of interference decrease with frequency, especially if highly directional antennas are used. On the other hand, higher frequency links, especially above 10 GHz, are subject to increasing rain fading. These trade-offs are especially noticeable at millimeter-wave frequencies. Two popular bands are the V-Band (around 59-66 GHz) and E-band (around 75-86 GHz). The V-band is license-free in the USA and several other countries, while the E-band is licensed with required frequency coordination. In both bands, the antenna beam-width can be maintained below 2 degrees, thus interference is highly unlikely, yet rain fading is quite significant. The range of a point-to-point link is determined in each geographical area based on the rain intensity statistics and the link's available fade margin. Typical industry accepted availability is 99.9% to 99.999% and a typical range at this availability is between 0.5 km-5 km.
Reducing the data rate can increase the range of a millimeter wave link, under fade-margin constraints, however the economical value of slower links decreases too. An acceptable compromise is to offer an adaptive-rate link, such as a GigE-link that reduces speed to FE under a strong rain fade. The link range may be determined so as to provide an availability of at least 99.99% with FE and at least 99.9% with GigE. It is desirable to provide such rate-adaptation with a minimum, or even no, interruption to service and to perform the rate switching without user intervention. Furthermore, if the Ethernet hierarchy is used, it is desirable to take advantage of available low-cost Ethernet chip-sets to minimize the cost of such rate-adaptive links, because the Ethernet protocols and the available chip-sets include rate adaptation functionality.
Some of the aspects of a digital millimeter wave radio link have been disclosed U.S. Pat. No. 6,937,666 which is assigned to the same assignee as the present application. The radio disclosed in the '666 Patent can provide further background information regarding millimeter wave digital radio links and their application.
When microwave links, especially in the millimeter bands, are used for implementing the backbone network of a campus or even metropolitan-area, it is desirable to offer drop/insert capabilities and Ethernet-based network-element management ports directly in the radio enclosure. Such implementation offers network flexibility by offering multiple services and cost reduction in equipment, installation and maintenance and improved service reliability.
A system and method that provides an adaptive rate digital microwave communications link and drop/insert capabilities is desirable and it is to this end that the present invention is directed.